[time-nuts] Re: Compairing the phase drifts between two PPS signals

[John Ackermann N8UR]

On 10/4/22 16:29, Mayukh Bagchi via time-nuts wrote:

Hi Mayukh!  See below...

>  From what I understand going through Thomas's (and John's) email and please correct me if I am wrong, I would be using our stable OCXO clock to run the TICC and then feed that same signal(10 MHz) into one of the input signal ports. In that case, would I also need to square the sine wave output of my OCXO? for that purpose would an LTC 6957 (with a CMOS logic output) be useful?
> And do I just insert the 1 PPS from the GPS into the other signal port of the TICC? Won't there be an issue as I am trying to measure two signals with different frequencies i.e 10MHz and 1 PPS? I guess I didn't quite understand that well.

There are two main ways to use the TICC for this kind of timing:  (a) as 
a traditional time interval counter with one DUT and one reference PPS 
input plus a 10 MHz clock input, or (b) as a timestamping counter with 
one (or two) PPS input, and a 10 MHz clock that also acts as the reference.

In case (a) you measure the (changing) interval between the two PPS 
sources, and the 10 MHz clock is used as a transfer standard -- its 
performance does not need to be anything special because you are using 
it to measure such short time periods that the clock noise is irrelevant 
(within reason).  You analyze the changes in time interval over time to 
determine frequency offset and stability.

In case (b) you measure a series of timestamps from one PPS source and 
compare the second-to-second variations of the PPS source compared to 
the 10 MHz clock.  In that case, the clock *is* important.  The 
timestamps might look like this (lots of decimal places removed for 
simplicity):

5.000 103
6.000 054
7.000 005
7.999 953
8.999 902

To extract phase data from this, you subtract the prior timestamp from 
the present one, and then subtract the nominal period to get the 
relative phase of the PPS compared to the 10 MHz clock, and analyze how 
that varies over time.  [ In this case, the TICC can measure timestamps 
from each of its two channels independently, so you can compare two 
oscillators against a common reference, if you want. ]

Case (a) is the traditional method and works just fine if you have two 
PPS sources; the 10 MHz source doesn't need to be anything too fancy. 
Case (b) is more convenient if you already have a 10 MHz output from 
your reference; it avoids a divider.  With the TICC, case (a) also gives 
you a two channel measurement system instead of just one.  In theory you 
can argue that the timestamp method might have slightly better 
performance than time interval, but in practice it's very hard to see 
the difference.

> Also, do you think overall the TICC would fit my case? Would I be able to measure the phase drift information well enough using this setup?

The TICC noise floor is something better than 1e-10 at tau=1 second and 
goes down decade per decade with increasing tau.  With the rule of thumb 
that the noise should be a decade below the measurement, you should be 
able to measure <1e-9 at 1 second with good confidence.  You'll have to 
decide whether that's sufficient.  If it's not, you'll probably need to 
go with a different and likely 17 dB more expensive measurement system.

Hope this helps!

John